An image forming apparatus includes a developing device that accommodates toner, and supplies the toner to a surface of a photoconductive drum. The developing device includes a mixer inside a casing. The developing device causes developer containing toner to circulate inside the casing using the mixer. End portions of the mixer are rotatably supported by the casing. There is some possibility that the developer in the casing may leak from the casing through the end portion of the mixer. When the developer leaks from the inside of the casing, a rotational failure of the mixer can occur as the leaked developer adheres to and then binds the moving portions of the mixer on the outside of the casing.